Time-gated fluorescence sensor for silver ions using Mn:CdS/ZnS quantum dots/DNA/gold nanoparticle complexes
A fluorescent sensor for the determination of Ag+ by time-gated mode is proposed. The method is based on Ag+-induced formation of quantum dot/DNA/gold nanoparticle complexes. Two ssDNA strands (strand A and strand B) are designed which contain C–C mismatched base pairs that can specifically react with Ag+. The water-soluble long-lifetime fluorescence quantum dots (Mn:CdS/ZnS) functionalized with strand A are selected as the fluorophore. The gold nanoparticles (GNPs) which are labeled with strand B, acted as the quencher. When Ag+ is absent in the sample solution, DNA-modified GNPs and Mn:CdS/ZnS were in the freedom state, the fluorescence signal of Mn:CdS/ZnS is obviously strong. When Ag+ are present in the sample solution, a DNA duplex is formed because of the strong binding ability between Ag+ and cytosine bases and formed stable C–Ag+–C structures. As a result, the Mn:CdS/ZnS and the GNPs are brought into close proximity, which caused the fluorescence quenching of the Mn:CdS/ZnS due to the nanometal surface energy transfer (NSET) between the Mn:CdS/ZnS and GNPs. This fluorescent sensor could present a satisfactory specificity and selectivity for Ag+. Meanwhile, the detection limit of Ag+ was estimated to be 7.9 nM.